1. Field of the invention
The present invention relates to a process for case-hardening or carburizing rolling bearing elements of low-alloy nickeliferous steel. In particular, the process of the present invention comprises the steps of producing a carburized layer by holding said rolling bearing elements in a carburizing furnace at a carburizing temperature of about 970.degree. C. in the presence of a carburizing gas which dissociates into carbon and hydrogen; cooling said rolling bearing elements; heating said cooled rolling bearing elements to hardening temperature; austenitizing said rolling bearing elements in the carburization layer; and quenching said austenitized rolling bearing elements to obtain a surface hardness of 60 to 64 RHC.
So that no brittleness of the rolling bearing elements takes place upon carburization due to dissociated hydrogen, the rolling bearing elements are cooled from the carburizing temperature of about 970.degree. C. down to a temperature of about 300.degree. C. in the presence of an agitated inert gas and then held at this temperature in the presence of said inert gas. The rate of cooling is such as to prevent precipitation of intercrystalline carbides in the steel structure and to cause a great deal, i.e., a substantial amount, of the dissociated hydrogen to diffuse out of the carburized layer of the rolling bearing elements. Subsequent heating of the said elements to hardening temperature or to intermediate annealing temperature is advantageously effected after holding at 300.degree. C.
2. Description of the prior art
Highly stressed rolling bearing elements, for example, large rolling bearing races for rolling mills, are frequently made of a low-alloy nickeliferous steel which is surface hardened by the case-hardening process. In this connection, after carburization and before austenitization of the carburized layer of the rolling bearing elements, intermediate annealing is often necessary for homogenizing the grain structure of the steel and for reducing internal stresses.
In a known process (U.S. Pat. No. 3,737,204) for case-hardening rolling bearing elements of the type mentioned, the rolling bearing elements, after carburization in propane gas, are slowly cooled down to room temperature, so that some of the hydrogen absorbed into the carburized layer due to dissociation of the propane gas during carburization is able to diffuse out of the carburized layer. This cooling, however, should not proceed too slowly, because otherwise intercrystalline carbides precipitate in the structure of the carbonized layer. Such precipitation alters the hardness structure and considerably reduces the rolling fatigue service life of the rolling bearing elements, for example, rolling bearing races.
In the known process some atomic and molecular hydrogen still remains in the carburized layer. In steels containing nickel, this hydrogen causes brittleness of the hardened structure of rolling bearing elements, which may lead to hazardous crack formation.